Mask pressure regulation in CPAP treatment and assisted respiration by dynamic control of mask vent flow

ABSTRACT

An air delivery system includes an air flow generator to provide a pressurized flow of air, a patient interface to provide a seal with the patient&#39;s face in use, an air delivery conduit to interconnect the air flow generator and the patient interface, and a controllable vent valve to control venting from the patient interface. The vent valve is controlled to maintain a substantially constant air flow in the air delivery conduit and the air flow generator.

CROSS-REFERENCE TO APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/887,537, filed Feb. 10, 2011, which is the U.S. National Phase ofInternational Application No. PCT/AU2006/000418, filed Mar. 29, 2006,which designates the U.S. and claims priority to Provisional ApplicationNos. 60/667,052, filed Apr. 1, 2005, 60/706,430, filed Aug. 9, 2005, andU.S. Pat. No. 60,775,334, filed Feb. 22, 2006, each incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to an air delivery system used fortreatment, e.g., of sleep disordered breathing (SDB) such as obstructivesleep apnea (OSA).

BACKGROUND OF THE INVENTION

Patients who suffer from obstructive sleep apnea can use continuouspositive airway pressure (CPAP) therapy to maintain the upper airwayopen while they are asleep. CPAP therapy is applied to the patient usinga mask, tubing, and a flow generator. All of these components encompassthe air delivery system provided to the patient.

One problem with CPAP therapy is that the flow impedance of a flowgenerator air delivery path results in pressure swings at the mask whenthe breathing load changes. As the impedance of the delivery path or therespiratory flow increases so will the observed pressure swings.

An existing solution to this problem includes reducing the inertia ofthe turbine or increasing peak power delivery from the power supply.These increase the ability of the system to compensate for changes inmask pressure. However, compensating for the pressure swings byadjusting the turbine pressure results in increased noise and bearingwear, and is fundamentally limited by the rate at which the turbinepressure can be changed, e.g., see U.S. Pat. No. 6,332,463.

Another existing solution to this problem includes constraining theproduct to have a low flow impedance delivery path, e.g., ResMed's S6device. However, maintaining a low flow impedance of the delivery airpath in a flow generator may compromise other product design goals,e.g., the turbine inlet air path being constrained to reduce radiatednoise, the delivery hose being reduced in diameter, or a humidifier orfilter being added to the patient circuit.

Under these conditions, it may be difficult to lower the pressure swingsat the mask to clinically acceptable levels and therapy delivery may becompromised.

Thus, there is a need for an improved system that does not suffer fromthe above-mentioned drawbacks.

SUMMARY OF THE INVENTION

One aspect of the invention relates to methods or systems used tocompensate for pressure swings at the entrance to the patient's airwayinduced by respiratory flow in the course of continuous positive airwaypressure (CPAP) treatment or assisted respiration.

Another aspect of the invention relates to an air delivery systemincluding an air flow generator to provide a pressurized flow of air, apatient interface to provide a seal with the patient's face in use, anair delivery conduit to interconnect the air flow generator and thepatient interface, and a controllable vent valve to control venting fromthe patient interface. The vent valve is controlled to maintain asubstantially constant air flow in the air delivery conduit and the airflow generator.

Another aspect of the invention relates to a method for deliveringpressurized air to a patient. The method includes delivering pressurizedair to a patient interface and controlling venting from the patientinterface to maintain a substantially constant air flow in the airdelivery conduit and the air flow generator.

Another aspect of the invention relates to a method for deliveringpressurized air to a patient. The method includes delivering pressurizedair to a patient interface and Controlling venting from the patientinterface to maintain a substantially constant pressure at the patientinterface.

Other, aspects, features, and advantages f this invention will becomeapparent from the following detailed description when taken inconjunction with the accompanying drawings, which are a part of thisdisclosure and which illustrate, by way of example, principles of thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings facilitate an understanding of the variousembodiments of this invention. In such drawings:

FIG. 1 is a schematic view of an air delivery system according to anembodiment of the present invention;

FIG. 2 is an embodiment of a control algorithm for the air deliverysystems shown in FIGS. 1 and 3;

FIG. 3 is a schematic view of an air delivery system according toanother embodiment of the present invention;

FIG. 4 is a schematic view of an air delivery system according to yetanother embodiment of the present invention; and

FIG. 5 is an embodiment of a control algorithm for the air deliverysystem shown in FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The air delivery methods or systems described below are alternatives tothe methods described in U.S. Pat. No. 6,332,463 to ResMed Limited, theentirety incorporated herein by reference. In the illustratedembodiments described herein, the air delivery methods or systemseliminate the effect of delivery circuit impedance on mask pressureswings without requiring a dynamic adjustment of the turbine speed.Because the methods or systems described below are not restricted byphysical response of a mechanical rotating element (i.e., a valve has afaster response time), they potentially allow the use of morerestrictive air paths.

Aspects of the air delivery methods or systems described below may beincorporated into other systems such as the Ventless Mask CPAP Systemdescribed in U.S. Patent Application No. 60/706,430, filed Aug. 9, 2005,the entirety incorporated herein by reference.

1. First Illustrated Embodiment of Air Delivery System

FIG. 1 illustrates an air delivery system 10 according to an embodimentof the present invention. The air delivery system 10 is structured todeliver a supply of pressurized breathable air to a patient fortreatment, e.g., of Sleep Disordered Breathing (SDB) with CPAP orNon-Invasive Positive Pressure Ventilation (NIPPV).

As illustrated, the air delivery system 10 includes an air flowgenerator 20, a patient interface 30, e.g., mask, and an air deliveryconduit 40.

The flow generator 20 includes an air turbine or blower 22 and a flowmeasuring element 24, e.g., a flow meter. The air turbine 22 is operableto draw a supply of air into the flow generator through one or moreintake openings and provide a pressurized flow of air at an outlet. Theair turbine 22 is driven by a controllable electric motor. The electricmotor is controlled by a constant pressure control 26 so as to deliverconstant elevated pressure (P_(turbine)) at constant flow(F_(delivery)). The constant pressure control 26 may be provided by anumber of mechanisms including a constant rpm control or a feedbackcircuit incorporating a pressure transducer, for example. The flowmeasuring element 24 provides a representation of the air flow throughthe flow generator (F_(delivery)). In the illustrated embodiment, theflow measuring device 24 is provided at the outlet of the flow generator20. However, the flow measuring device 24 may be located at any point inthe air path of the flow generator 20, e.g., either before or after theair turbine 22.

The supply of pressurized air is delivered to the patient via the airdelivery conduit 40 that includes one end communicated to the outlet ofthe flow generator 20 and an opposite end communicated to the patientinterface 30. The air delivery conduit 40 may have any suitableconstruction, and may be communicated or coupled to the flow generator20 and the patient interface 30 in any suitable manner. The pressuredrop between the flow generator 20 and the patient interface 30 atconstant flow may be characterized as (P_(drop)).

The patient interface 30 comfortably engages the patient's face andprovides a seal so that pressurized air (P_(mask)) can be delivered tothe patient. The patient interface 30 may have any suitableconfiguration as is known in the art, e.g., full-face mask, nasal mask,oro-nasal mask, mouth mask, nasal prongs, etc. The patient interface ormask 30 is equipped with two ports 32, 34. The first port 32 is an inletport for the delivery of air (e.g., via air delivery conduit 40), andthe second port 34 is an outlet port for the venting of exhaled air andsurplus air flow. The patient air flow (F_(patient)) will consist ofboth respiratory air flow components (F_(resp)) and non-respiratory airflow components including mask leak (F_(nonresp)).

The vent flow (F_(vent)) from the outlet port 34 is controlled by acontrollable proportional valve or vent valve 50 in such a manner so asto maintain a constant delivery flow (F_(delivery)) for all normalpatient flow conditions. In the illustrated embodiment, the vent valve50 is external to the flow generator 20. The vent flow (F_(vent)) isindirectly modulated by patient respiration, i.e., decreasing duringinspiration and increasing during expiration.

The vent valve 50 is controlled by a constant flow control 52. Theconstant flow control 52 receives feedback from the flow measuringelement 24 for (F_(delivery)) and selectively adjusts the vent valve 50for (F_(vent)) to maintain constant flow.

1.1 Constant Regulation of Mask Pressure

The constant delivery flow is selected to be the maximum patient flowexpected under normal conditions(F_(delivery)=max(F_(resp)+F_(nonresp))). In one example, this would be75 l/min.

The vent flow required to maintain this constant flow is given by(F_(vent)=F_(delivery)−F_(resp)−F_(nonresp)). In an embodiment, thiswould be in the range 0-125 l/min. The vent flow varies as therespiratory flow varies in order to maintain constant mask pressure.Thus, if the mask pressure is constant and the flow generator pressureis constant, then the inlet flow would also be constant.

The mask pressure is a function of the turbine pressure and the pressuredrop in the air delivery conduit (P_(mask)=P_(turbine)−P_(drop)). Atconstant flow, the turbine pressure and the pressure drop are constant,and so the air delivery system 10 provides a constant, regulation ofmask pressure.

1.2 Control Algorithm

FIG. 2 illustrates a control algorithm for the air delivery system 10 ofFIG. 1. The algorithm is preferably implemented using software, althoughhardware implementations are also possible. As illustrated, the measuredair flow is obtained from the flow measuring element 24 at step 201. Atarget air flow, i.e., constant air flow, is established at step 202,and the measured air flow is compared to the target air flow at step203. If the measured air flow is above the target air flow, then thevent valve 50 is opened further via control 52 at step 204. If themeasured air flow is not above the target air flow, then the vent valve50 is adjusted towards the closed position via control 52 at step 205.The process continues through operation of the air delivery system 10 tomaintain constant flow.

2. Second Illustrated Embodiment of Air Delivery System

FIG. 3 illustrates an air delivery system 200 according to anotherembodiment of the present invention. Similar to the air delivery system10, the air delivery system 200 includes an air flow generator 220, apatient interface 230, e.g., mask, and an air delivery conduit 240. Theflow generator 220 includes an air turbine 222 controllably driven by aconstant pressure control 226, and a flow measuring element 224. Thepatient interface or mask 230 is equipped with inlet and outlet ports232, 234. A vent valve 250 is controlled by a constant flow control 252to control the vent flow from the outlet port 234.

The air delivery system 200 includes several enhancements or variationswith respect to the air delivery system 10. For example, one enhancementis that the vent valve 250 is physically located in the flow generator220. Such an arrangement may be advantageous in simplifying the controlof the vent flow and isolating any mechanical noise associated with thevent valve 250 from the patient. In order to locate the vent valve 250within the flow generator 220, a return vent conduit 242 is providedbetween the vent valve 250 and the outlet port 234. Pressure regulationat the patient interface 230 may be retained if the pressure drop acrossthe return vent conduit 242, and the vent valve 250 at maximum vent flowdoes not exceed the minimum mask pressure. If the pressure drop acrossthe return circuit cannot be reduced sufficiently to maintain pressureregulation, then a partial vacuum may be applied to the exhaust port ofthe vent valve 250. This will compensate for some of the pressure dropin the return circuit and allow pressure regulation to be restored.

Another enhancement is that the conduit connections to the patientinterface 230 include one-way valves in order to prevent reverse airflow from the patient to the flow generator for the purpose of reducingcross-infection. Specifically, the inlet port 232 communicated with airdelivery conduit 240 includes one-way valve 260 that ensures thatexhaled air cannot enter the air delivery path. The outlet port 234communicated with the return vent conduit 242 includes one-way valve 262that ensures that inhaled air cannot be taken from the vent. In normaloperation, the air flow will pass from the air delivery path to thevent. In abnormal respiratory flows (e.g., coughs and gasping), the airflow may momentarily result in a flow reversal in either conduit 240,242. The one-way valves 260, 262 prevent such reverse air flows, therebyreducing the possibility of patient contamination entering the deliveryair path. The constant flow out of the flow generator would tend toprevent reverse air flow from happening, i.e., there is very littlechance of expired air entering the flow generator.

Yet another enhancement is that the mask pressure may be controlled,possibly in synchronism with the respiratory flow, for the purpose ofassisting respiration, improving comfort, and/or adapting therapy. Forexample, a basic representation or flow profile 254 of the respiratoryflow can be obtained from the flow control 252 associated with the ventvalve 250. The fidelity of the respiratory flow representation may beimproved by the addition of sensors, either measuring the pressure dropacross the vent valve 250 or directly measuring the flow through thevent valve 250. The respiratory flow is derived from the vent flow afterremoving non-respiratory flow components(F_(resp)=F_(delivery)−F_(vent)−F_(nonresp)). The flow profile 254 fromthe position of the vent valve 250 may be used to determine therespiratory flow to trigger the pressure profile 228.

The mask pressure profile 228 may be varied by applying the requiredwaveform (increased by the pressure loss in the air delivery conduit240) as the turbine pressure control set point. Because the pressureloss in the air delivery conduit 240 is constant at constant flow, themask pressure will faithfully follow any applied pressure profile 228.

2.1 Control Algorithm

The control algorithm for the air delivery system 200 of FIG. 3 isbasically the same as the air delivery system 10 of FIG. 1 (see FIG. 2).As illustrated, the measured air flow is obtained from the flowmeasuring element 224 at step 201. A target air flow, i.e., constant airflow, is established at step 202, and the measured air flow is comparedto the target air flow at step 203. If the measured air flow is abovethe target air flow, then the vent valve 250 is opened via control 252at step 204. If the measured air flow is not above the target air flow,then the vent valve 250 is closed via control 52 at step 205. Theprocess continues through operation of the air delivery system 200 tomaintain constant flow.

3. Third Illustrated Embodiment of Air Delivery System

FIG. 4 illustrates an air delivery system 300 according to yet anotherembodiment of the present invention. As illustrated, the air deliverysystem 300 includes an air flow generator 320, a patient interface 330,e.g., mask, and an air delivery conduit 340.

The flow generator 320 includes an air turbine 322 that is set tooperate at a specified power input by a constant power control 325.Alternatively, the air turbine 322 may be operated by a constant rpm orconstant pressure controller. The air delivery conduit 340 includes oneend communicated to the outlet of the flow generator 320 and an oppositeend communicated to the inlet port 332 of the patient interface 330. Avent or exhaust valve 350 is preferably physically located in the flowgenerator 320, and a return vent conduit 342 is provided between thevent valve 350 and the outlet port 334 of the patient interface 330. Thevent valve 350 is controlled by a valve control 351 to control the ventflow from the outlet port 334. In use, the valve control 351 adjusts thevent valve 350 so as to maintain constant pressure at the patientinterface 330 (this will alter the flow in the air delivery circuit).Alternatively, a constant rpm or constant pressure controller maymaintain constant pressure at the patient interface 330.

3.1 Adaptive Flow

As illustrated, an adaptive flow control 370 monitors the setting of thevent valve 350 and adjusts the specified power input at the constantpower control 325 as required to minimize the air flow. That is, the‘constant’ air flow in the air delivery circuit may be adjustedgradually by the adaptive flow control 370 so as to establish a Minimumconsistent with the delivery of therapy. This optimization of the airflow is termed “adaptive flow.” The operating limits of the vent valve350 are monitored over time and the air delivery flow may be adjusted bychanging the turbine pressure as shown below:

Vent Valve Position Air Delivery Flow Adaptive Action Often fully openToo high Reduce turbine pressure or power or rpm Often fully closed Toolow Increase turbine pressure or power or rpm

Optimization of the air flow has several advantages. For example,reduction in average air flow reduces power consumption of the airturbine and reduces noise. Also, the operating range of the vent valveis maintained ensuring maximum control resolution. In addition,operating lifetime of the system may be increased.

When implemented, the vent valve 350 should not be fully closed (in asystem with no fixed mask leak) as a minimum vent or exhaust flow willbe needed to ensure mask CO₂ flush-out.

3.2 Introduction of Pressure Sensing at Mask

The pressure at the patient interface 330, e.g., mask, may be senseddirectly at the patient interface 330. Pressure sensing at the patientinterface 330 may be implemented through a pressure sensor mounted on oradjacent to the patient interface 330 (possibly in conjunction with acontrollable vent valve) or a pressure sensor located in the flowgenerator and communicated with the patient interface via a pressuresensing tube.

In the illustrated embodiment, the pressure sensor 380 is located in theflow generator 320, and a pressure sensing tube 382 is provided betweenthe pressure sensor 380 and the patient interface 330. In use, thepressure at the patient interface 330 may be monitored remotely by thepressure sensor 380 through the pressure sensing tube 382.

Other sensing arrangements for sensing pressure at the patient interfaceare described in U.S. Patent Application No. 60/731,483, entitled“Sensing Cuff for Breathing Apparatus,” filed Oct. 31, 2005, theentirety incorporated herein by reference.

In previous embodiments, mask pressure is computed as a function ofmeasured air flow and measured or computed turbine pressure. Pressuresensing at the patient interface removes the need for this computation.

Pressure sensing at the patient interface has several advantages. Forexample, air flow measurement does not need to be calibrated. The onlyindication of air flow needed is that sufficient to maintain the airflow constant. Also, operation is not affected by changes in impedanceof the air delivery path, such as changes to patient circuit componentsor the gradual change in impedance of a humidifier.

3.3 Establishing ‘Constant’ Air Flow Delivery

Air delivery flow may be set without reference to a measured flow value.If impedance in the air delivery path remains constant, then maintaininga constant turbine rpm or turbine energy input will result in constantair flow. This is advantageous as it eliminates the need for an air flowsensing element.

In the absence of a calibrated air flow measurement, the autosettingflow algorithm (described below) would require the establishment of aninitial condition from which it would then monitor vent valve positionand optimize the air flow. In an embodiment, the pulse-width modulation(PWM) or energy input to the air turbine may be set relatively high toensure that therapeutic mask pressure is available.

3.4 Fixed Flow Turbine Design

A fixed flow turbine design allows optimization of the air turbine fornoise reduction and/or size reduction. If the air turbine rpm remainsconstant, then noise components due to rpm shift are eliminated. Even ifa variable flow is implemented (e.g., as in adaptive flow describedabove), then the air turbine may be optimized for the air flow with zeromask leak as this would be the normal operating condition.

Prior art systems often require rising fan curve turbine designs (i.e.,turbines that deliver higher pressure as the flow increases, at anessentially fixed rpm). According to an aspect of the present invention;keeping flow constant (or nearly constant) removes this constraint fromthe turbine design. Allowing, for example, fan curves that “fall” (i.e.,that give far less pressure with increasing flow, at a fixed rpm) leadto flexibility in the turbine design, such that the turbine can beoptimized for low noise and/or small size.

3.5 Mask Diffuser

In an embodiment, the patient interface or mask may have a diffuser, inaddition to the inlet and outlet ports. The diffuser may be set togenerate a required exhaust flow at 3 cm. As the mask pressureincreases, a significant flow occurs through the diffuser in alternativedesigns. This flow is waste flow that has an energy cost. Each of theabove-described embodiments are characterized by a controlled minimumflow that does not increase with increased mask pressure, which is anadvantage. If the mask has a diffuser and the mask pressure varies, thenthe diffuser flow varies. This is an advantage of not having a diffuserin this design, i.e., the vent valve may be used to set a minimum flowout of the mask to ensure good CO₂ washout.

3.6 Delivery of Autosetting Pressure Therapy

In the illustrated embodiment, the setting of the vent valve 350 may befiltered to remove any DC component (representing mask leak). Theresulting patient flow profiles may be used by an autosetting algorithmto adjust the mask pressure.

Specifically, a basic representation or flow profile 390 of therespiratory flow can be obtained by the filtered setting or position ofthe vent valve 350. The flow profile 390 may be used as an input to anautosetting algorithm 392. The autosetting algorithm 392 adjusts themask pressure set point 394 via the valve control 351.

Examples of autosetting algorithms for delivering autosetting therapy(e.g., auto-titration) are described in U.S. Pat. Nos. 5,704,345 and6,363,933, each of which is incorporated herein by reference in itsentirety.

This arrangement provides a mechanism to deliver autosetting therapywithout a flow sensor.

3.7 Measurement of True Leak and Patient Flow

The air flow in the air delivery conduit 340 and the return vent conduit342 may be determined by pressure sensors. By placing a pressure sensorat each of the flow generator, the patient interface, and before thevent valve, the air flow in each conduit 340, 342 may be determined froma knowledge of the impedance of the air flow path. That is, subtractingthe exhaust air flow from the delivery air flow yields the instantaneouspatient air flow. A low pass filter of the patient air flow provides themask leak.

3.8 Impact on Humidification

The air delivery systems described above may simplify humidifier designbecause the constant air flow yields more consistent humidification.That is, the reduction in air flow that arises out of the adaptive flowcontrol and the removal of diffuser leak means that the humidificationburden may be reduced, particularly at high pressures.

3.9 Control Algorithm

FIG. 5 illustrates a control algorithm for the air delivery system 300of FIG. 4. The algorithm is preferably implemented using software,although hardware implementations are also possible. As illustrated, thepatient interface pressure or mask pressure is obtained from thepressure sensor 380 at step 501. A target pressure, i.e., the maskpressure set point 394, is established at step 502, and the measuredmask pressure is compared to the target pressure at step 503. If themask pressure is above the target pressure, i.e., above the maskpressure set point 394, then the vent valve 350 is opened further viavalve control 351 at step 504. If the mask pressure is not above thetarget pressure, i.e., not above the mask pressure set point 394, thenthe vent valve 350 is adjusted towards the closed position via valvecontrol 351 at step 505. A new valve position is established at step506.

If the new position of the vent valve 350 is nearly fully closed at step507, then the power to the air turbine 322 is gradually increased viaadaptive flow control 370 at step 508. If the new position of the ventvalve 350 is nearly fully open at step 509, then the power to the airturbine 322 is gradually decreased via adaptive flow control 370 at step510. If the new position of the vent valve 350 is neither nearly fullyclosed or open, then the process returns to obtaining the mask pressureat step 501. The process continues through operation of the air deliverysystem 300 to maintain constant flow.

4. Advantages

The air delivery systems described above may have specific advantages tothe patient, physician, and/or manufacturer.

One advantage is that the air delivery systems described above mayprovide a system that is more comfortable and easier to use for thepatient. For example, the relaxation of restrictions on delivery circuitresistance allows a more flexible design approach to the delivery/ventconduits. Also, because the air flow through any humidifier in the airdelivery circuit is constant, the temperature and humidity of the airwill not be affected by patient breathing. This uniformity may havebenefits that outweigh the extra humidification required because theaverage air flow through the humidifier is higher.

Another advantage is that the air delivery systems described above maybe easier to administer for the physician. For example, the constantflushing of the mask should result in lower levels of CO₂ and lessrebreathing. With valves incorporated in the mask conduits, there wouldstill be a flushing of clean air with the flow generator turned off.Further, the positive air flow through the flow generator reduces thepossibility of patient contamination of the device. This arrangement maybe an attractive alternative to disposable air paths in the flowgenerator.

Yet another advantage is that the air delivery systems described abovemay be cheaper for the manufacturer. For example, it may be cheaper toimplement an air turbine and motor that does not require a low inertiaand a humidifier that operates with a fixed air flow. Also, therelaxation of constraints on the delivery path resistance allows morecost effective approaches to noise reduction. Further, the reduction inmotor rpm may increase the bearing lifetime and reduce operatingtemperatures.

Another advantage is that the control element of the system, i.e., ventvalve, is in the exhaust path, so it does not come in contact with thepatient air supply. This arrangement permits the use of alternativematerials for the vent valve and sound absorbing. Further, theabove-described systems do not require an accurately calibrated vent aspart of the mask. In addition, the above-described systems prevent theuse of competitor masks that do not have a second vent port.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the invention. Also, the various embodiments described abovemay be implemented in conjunction with other embodiments, e.g., aspectsof one embodiment may be combined with aspects of another embodiment torealize yet other embodiments. In addition, while the invention hasparticular application to patients who suffer from OSA, it is to beappreciated that patients who suffer from other illnesses (e.g.,congestive heart failure, diabetes, morbid obesity, stroke, barriatricsurgery, etc.) can derive benefit from the above teachings. Moreover,the above teachings have applicability with patients and non-patientsalike in non-medical applications.

The invention claimed is:
 1. A system for delivering pressurized air toa patient, the system comprising: a patient interface configured tocommunicate the pressurized air to the patient; an air source configuredto deliver the pressurized air to the patient interface; and a ventvalve configured to control venting from the patient interface tomaintain a substantially constant pressure at the patient interface,wherein the patient interface is between the air source and the ventwith respect to a direction of air flow from the air source to the vent.2. The system according to claim 1, wherein the air source comprises anair flow generator.
 3. The system according to claim 2, wherein the airflow generator includes a blower that is driven by a controllableelectric motor.
 4. The system according to claim 3, further comprising aconstant pressure control configured to control the electric motor. 5.The system according to claim 4, wherein the constant pressure controlincludes a feedback circuit with a pressure transducer.
 6. The systemaccording to claim 3, further comprising a constant speed controlconfigured to control the electric motor.
 7. The system according toclaim 2, wherein the air flow generator includes a flow measuringelement to provide a representation of air flow through the air flowgenerator.
 8. The system according to claim 2, wherein the vent valve islocated external to the air flow generator.
 9. The system according toclaim 2, wherein the vent valve is located within the air flowgenerator.
 10. The system according to claim 9, wherein the vent valveis communicated with the patient interface by a return vent conduit. 11.The system according to claim 1, wherein the vent valve is configured tovary flow rate through the vent valve to maintain the substantiallyconstant pressure.
 12. The system according to claim 11, wherein thesystem is configured to vary the flow rate through the vent valve asrespiratory flow varies to maintain the substantially constant pressure.13. The system according to claim 1, wherein the patient interfaceincludes an inlet port communicated with an air delivery conduit for airdelivery from the air source and an outlet port communicated with thevent valve for air venting.
 14. The system according to claim 13,wherein the inlet port includes a one-way valve to prevent reverse airflow from the patient to the air source.
 15. The system according toclaim 1, further comprising a controller configured to calculatepressure at the patient interface as a function of measured air flow andmeasured or calculated pressure at the air source.
 16. The systemaccording to claim 1, wherein system is configured so that all CO2washout occurs through the vent valve.
 17. A method for deliveringpressurized air to a patient, the method comprising: deliveringpressurized air to a patient interface from an air source; and using avent downstream from the patient interface, controlling venting from thepatient interface to maintain a substantially constant pressure at thepatient interface while flow out of the vent is at or above apredetermined minimum flow rate.
 18. The method of claim 17, wherein theair source is a flow generator, and further comprising controlling theflow generator to output a substantially constant pressure.
 19. Themethod of claim 17, wherein the air source is a flow generator with anelectric motor, and further comprising controlling the flow generator sothat the electric motor operates at a substantially constant rpm. 20.The method of claim 17, wherein the air source is a flow generator, andfurther comprising controlling the flow generator to use substantiallyconstant power to pressurize the air.
 21. The method of claim 17,further comprising monitoring the venting and selectively controllingdelivered air to establish a minimum air flow sufficient for CPAPtherapy.